The determination of the UHECR energy in both Auger and TA is obtained by calibrating the fluorescence detection given the EAS fluorescence yield expected for a given energy of the primary particle. The ways Auger and TA implement this technique show a relevant difference. While Auger uses the absolute light yield and its wavelength behaviour as measured by the Airfly Collaboration (see [1]}, [2]} and references therein), TA uses the absolute florescence yield measured at fixed wavelength (337 nm) by Kakimoto et al. and the wavelength behaviour of the yield as determined by FLASH (see [1]}, [2]} and references therein). Another important difference between the two experimental techniques is related to the way the invisible energy (i.e. the portion of EAS energy not detectable) is treated. In the case of Auger this estimation is based on the Airfly measurements, while in the case of TA is obtained through MC simulations [1]}, [2]}.
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